There is described herein a turbine blade having a tip shroud with cooling circuits for use in high temperature applications.
Turbine blade tip shrouds can be used to provide a useful flowpath shape (conical flowpath outer diameter) and to minimize tip leakage flow to increase turbine efficiency. Tip shrouds can also provide structural benefits by changing blade natural frequencies and mode shapes, as well as providing frictional damping from the interaction between mating blade shroud segments. Tip shrouds can degrade in operation by creep (curling up of shroud edges) or oxidation if the shroud metal temperature and/or stress exceed the capability of the material from which the blade and the shroud are produced.
Historically, it has been difficult and expensive to provide cooling features to turbine blade tip shrouds. As a result, blades with tip shrouds often have been limited to lower temperature stages of a gas turbine engine. Limitations in manufacturing capability have greatly constrained shroud cooling features, with existing designs either providing lightweight, extensive cooling at great cost, simple cooling at reduced cost or thick, heavy designs which require very heavy blades and rotors to support the large cooled shrouds.
Use of traditional ceramic core materials to form internal cooling passages in blade shrouds results in air passages which are excessively thick compared to the rest of the shroud geometry, leading to an excessively thick and heavy blade tip and a very heavy blade/rotor stage. Failure can occur due to the high stress imparted by the heavy tip shroud.
Other methods used in the past are open cavities closed with coverplates, such as that shown in FIG. 1. The coverplates are welded over machined cooling passages. The coverplates tend to be heavy and the overall process of manufacture is expensive.
Another method used in the past is the fabrication of EDM cooling passages. Such a method is shown in FIG. 2. Forming cooling passages in this manner is expensive and has very limited, straight line passage geometry limitations.
These prior processes for forming shrouds with cooling are expensive, create life debits due to welding, and can form heavy shrouds due to parasitic mass of a coverplate. Still other processes are slow as well as expensive and provide limited cooling passage geometry capability.
FIGS. 3-5 show a large-size industrial engine airfoil concept that uses a large plenum core in the tip shroud fed by drilled holes in the blade. The dashed outline shown in FIG. 5 illustrates the plenum boundary. The airfoil is fabricated using covers and ceramic core inserts. This fabrication concept suffers from being expensive and heavy. Further, this concept used a plenum, rather than a defined duct with a confined path with inlets and exits. Plenums such as this suffer from uncertain local internal flow conditions with low heat transfer.